In some applications, in particular the steering systems for the rear wheels of a four-wheel steering vehicle, electric steering actuators are favored because of their desirable power characteristics and compact and light-weight design. For instance, the electric steering actuator disclosed in U.S. Pat. No. 5,330,023 comprises an electric motor having a hollow output shaft, a steering shaft passed through the hollow center of the output shaft of the electric motor, and a ball-screw mechanism provided between the output shaft and the steering shaft for converting the rotary movement of the output shaft into a longitudinal movement of the steering shaft.
The ball-screw mechanism comprises a threaded section of the steering shaft, and a nut member which threadably engages with the threaded section of the steering shaft via steel balls so that the rotary movement of the nut member is converted into an axial movement of the steering shaft. The ball-screw mechanism inevitably transmits some torque from the nut member to the steering shaft, due to friction and other causes, and it is therefore necessary to restrict the rotary movement of the steering shaft around its axial line while permitting its axial movement. According to this previous proposal, a radial arm extending from the steering shaft is guided along a guide slot defined in the casing of the actuator.
However, if there is any significant play between the radial arm and the guide slot, a knocking noise is produced each time the direction of movement of the steering rod is reversed. Therefore, this guide mechanism is required to be substantially free from play, and machining of the radial arm and the guide slot must be carried out at a high precision. This complicates the manufacturing process, and increases the manufacturing cost.